The present invention relates to a scanning apparatus to scan an object by means of a light beam comprising at least one mirror, which, during operation, is driven by driving means to make a combined movement of rotation around an axis of a continuous rotation and an oscillating movement relative to the axis of rotation.
Such a scanning apparatus is known form GB-A-2.255.650, which shows various scanning devices able to produce generally parallel scanning lines on an object to be scanned. In all the scanning devices shown, at least one tiltable mirror is used. In one embodiment (FIG. 10) a rotatable polygon having mirror facets is shown, which is driven in such a way that, during operation, each of the facets is not only made to rotate but also made to oscillate, since the entire polygon is made to oscillate during its rotation. However, in the known device the oscillating movement of the entire polygon is coupled to the rotating movement. So, the light beam incident upon each of the facets will generate a fixed, curved scanning line. This scanning apparatus is not able to generate a helical scanning pattern or sections thereof, in which the pitch of the pattern, i.e. the mutual distance between neighbouring scanning lines of the pattern may be controlled. Moreover, no change of direction of the generated scanning lines is possible.
EP-A-0.456.095 discloses the application of a mirror in a bar code reading device, which mirror is made to oscillate in two perpendicular directions. The arrangement according to FIG. 3 of this European patent application is the one most close to the present patent application. The mirror can be caused to rotate by a motor and can be caused to oscillate by electromagnetic driving means. However, the output shaft of the motor does not continuously rotate but oscillates in alternate directions. Therefore, the output shaft to which the mirror is connected is not able to rotate in one direction only. The rotating movement of output shaft is restricted by a torsion spring connected to the other end of output shaft allowing only an oscillating alternating movement of the output shaft.
U.S. Pat. No. 4,838,632 discloses a two-dimensional beam scanner in which a mirror is able to be moved in two perpendicular directions. The mirror is pivotable about a first axis A and can be rotated about a second axis B perpendicular to the axis A by a stepper motor which is able to drive the mirror about the axis B in alternate directions. The stepper motor only drives the mirror in a stepping mode by equal angular steps. Therefore mirror is not driven continuously in one direction only about axis of rotation B. Moreover, there is only shown a scanning plane of limited dimensions (FIG. 3), which implies that the total amount of rotation about axis B is limited and that the rotation will be in alternate directions only, as indicated by a double arrow in the figures. No cylindrical scanning plane surrounding the scanner is either indicated or suggested. Besides, in the description of U.S. Pat. No. 4,838,632 it is not explained how the mirror can be oscillated about its pivot axis A; the only indication given is that such an oscillating movement may done by "any means known in the art" and that the mirror is typically a "galvanometer mirror" (column 2, lines 66-68), whereas causing a mirror to make a combined movement of two single perpendicular movements is not easy.